Finished cleanup of util's interaction with spectrogram.  min_delt was used as both a function call and a local variable in spectrogram and the function was never used outside, so the import of min_delt from util was replaced by the local definition of _min_delt.  This also serves to differentiate the function call from the variable name.
Finished cleanup of callisto's interface with sunpy.util.  Made callisto test import sequence PEP 8 compliant
PEP 8 discourages relative imports.
Finished cleanup of sunpy.util public interface